The present invention relates to a method of manufacturing parts for use in a heating furnace which is employed in a heating process such as oxidation and diffusion of semiconductor wafers in the course of manufacturing the semiconductor devices and, more particularly, it relates to a method of manufacturing a process tube for a diffusion furnace.
Quartz glass and silicon carbide are well known as materials of which the process tube for the diffusion furnace is made. When using quartz glass, a process tube of high quality can be obtained, but it becomes devitrified and bent under high temperature. Therefore, it is difficult to use this process tube under a temperature higher than 1,200.degree. C. In addition, it is also difficult to make a process tube large-sized, thereby restraining semiconductor wafers from being made large-sized. On the other hand, a process tube made of silicon carbide has good mechanical strength under high temperature, but the tube must be impregnated with silicon to make the tube gas-impenetrable. When impregnated with silicon, impurities in the tube diffuse and penetrate outside through the phase of silicon impregnated to thereby contaminate the silicon wafers which are arranged in the process tube at the diffusion process.
Further, the conventional process tube made of silicon carbide for use in the diffusion furnace is by no means satisfactory from the viewpoint of impurities. Namely, the conventional process tube made of silicon carbide is usually prepared after acid treatment, molding and purification treatment of silicon carbide powder as raw material, impregnation of high purity Si, and final acid treatment. However, it cannot be avoided that impurities are still left in the final product even if the cleaning is repeated several times as described above, and these impurities degraded the semiconductor device obtained. It was particularly difficult to remove those impurities which were confined in silicon carbide powder particles.
In order to remove these impurities, treatment was carried out at a temperature of 1,200.degree.-1,300.degree. C. for more than 100 hours at the gas purge step prior to the treatment of the silicon wafers, using a gas mixture of 3% HCl and O.sub.2. This treatment represented a large expense and loss of time.